Python tetris input delay gets exponentially longer

import turtle
import random

wn = turtle.Screen()
tetris = turtle.Turtle()
wn.tracer(0)
grid = []
tetriminos = ['t','l','j','i','s','z','o']

class square():
    def __init__(self, falling, block, color):
        self.falling = falling
        self.block = block
        self.color = color
for i in range(12):
    grid.append([])
for i in range(12):
    for j in range(25):
        grid[i].append(square(False, False, 'gray'))

def gridsquare(color):
    tetris.setheading(0)
    tetris.color(color)
    tetris.begin_fill()
    for i in range(4):
        tetris.forward(20)
        tetris.right(90)
    tetris.end_fill()
    tetris.color('black')

def refreshGrid():
    for i in range(12):
        for j in range(25):
            tetris.pu()
            tetris.goto(i * 20 - 120, j * 20 - 250)
            tetris.pd()
            if grid[i][j].block == True:
                gridsquare(grid[i][j].color)
            elif grid[i][j].falling == True:
                gridsquare('green')
            else:
                gridsquare('gray')
    wn.update()

def left():
    ok = True
    for i in range(12):
        for j in range(25):
            if grid[i][j].falling == True:
                if grid[i - 1][j].block == True:
                    ok = False
    if ok == True:
        for i in range(12):
            for j in range(25):
                if grid[i][j].falling == True:
                    grid[i][j].falling = False
                    grid[i - 1][j].falling = True
        refreshGrid()

def right():
    ok = True
    for i in range(12):
        for j in range(25):
            if grid[i][j].falling == True:
                if grid[i + 1][j].block == True:
                    ok = False
    if ok == True:
        for i in range(12):
            for j in range(25):
                if grid[11 - i][j].falling == True:
                    grid[11 - i][j].falling = False
                    grid[11 - i + 1][j].falling = True
        refreshGrid()

def down():
    ok = True
    for i in range(12):
        for j in range(25):
            if grid[i][j].falling == True:
                if grid[i][j - 1].block == True:
                    ok = False
    if ok == True:
        for i in range(12):
            for j in range(25):
                if grid[i][j].falling == True:
                    grid[i][j].falling = False
                    grid[i][j - 1].falling = True
        refreshGrid()
    else:
        for i in range(12):
            for j in range(25):
                if grid[i][j].falling == True:
                    grid[i][j].falling = False
                    grid[i][j].block = True
                    grid[i][j].color = 'blue'
        refreshGrid()
        newTetrimino()

def newTetrimino():
    tet = tetriminos[random.randint(0, 6)]
    #'t','l','j','i','s','z','o'
    #middle top - 5, 24
    if tet == 'z':
        grid[5][24].falling = True
        grid[5][23].falling = True
        grid[4][24].falling = True
        grid[6][23].falling = True
    elif tet == 's':
        grid[5][24].falling = True
        grid[5][23].falling = True
        grid[4][23].falling = True
        grid[6][24].falling = True
    elif tet == 't':
        grid[5][23].falling = True
        grid[4][23].falling = True
        grid[6][23].falling = True
        grid[5][24].falling = True
    elif tet == 'l':
        grid[5][23].falling = True
        grid[4][23].falling = True
        grid[6][23].falling = True
        grid[6][24].falling = True
    elif tet == 'j':
        grid[5][23].falling = True
        grid[4][23].falling = True
        grid[6][23].falling = True
        grid[4][24].falling = True
    elif tet == 'i':
        grid[5][24].falling = True
        grid[4][24].falling = True
        grid[6][24].falling = True
        grid[7][24].falling = True
    elif tet == 'o':
        grid[6][24].falling = True
        grid[7][24].falling = True
        grid[6][23].falling = True
        grid[7][23].falling = True
    refreshGrid()

grid[5][23].falling = True
grid[4][23].falling = True
grid[6][23].falling = True
grid[5][24].falling = True
for i in range(25):
    grid[0][i].block = True
    grid[0][i].color = 'black'
for i in range(25):
    grid[11][i].block = True
    grid[11][i].color = 'black'
for i in range(12):
    grid[i][0].block = True
    grid[i][0].color = 'black'

refreshGrid()
wn.listen()
wn.onkeypress(left,'a')
wn.onkeypress(right,'d')
wn.onkeypress(down,'s')
wn.mainloop()

Ok so Sorry for long code I'm trying to code tetris and i have the basics down but for some reason each tetrimino you place, it gets exponentially longer input delay I expect a little bit of input delay because of how much it has to check but i don't see why there gets more after each placement

help would be appreciated a lot

Also I am very much a beginner so please explain it as if I was an old grandma thanks